LeetCode算法题——ZigZag Conversion

Description:
The string 

"PAYPALISHIRING"

 is written in a zigzag pattern on a given number of rows
like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: 

"PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string text, int nRows);

convert("PAYPALISHIRING", 3)

 should
return 

"PAHNAPLSIIGYIR"

Zigzag:即循环对角线结构（

0				8				16
1			7	9			15	17
2		6		10		14		18
3	5			11	13			19
4				12				20

）

算法思想：
向下循环
斜对角循环：gap=nRows-2(去除端点)

C++ Implements:
string convert(string s, int numRows) {
        if(numRows==1) return s;
        string str[numRows];
        int i=0,gap=numRows-2;
        while(i<s.size()){
            for(int j=0;j<numRows&&i<s.size();j++){
                str[j]+=s[i++];
            }
            for(int j=gap;j>0&&i<s.size();j--){
                str[j]+=s[i++];
            }
        }
        string res="";
        for(int j=0;j<numRows;j++){
            res+=str[j];
        }
        return res;
    }